<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      * {
        margin: 0;
        padding: 0;
        /* 所有的盒子都会采用c3的盒子模型 */
        box-sizing: border-box;
      }
      .box {
        width: 200px;
        height: 200px;
        background-color: #f00;
        padding: 20px;
      }
    </style>
  </head>
  <body>
    <div class="box"></div>
  </body>
</html>
